The classic e-beam system is a "probe-forming" system in which a narrow beam that is the image of the electron source and has a gaussian distribution is scanned over the wafer or other target by an amount equal to a "pixel" at a time, the pixel being defined as the full width at half height of the intensity distribution. Such "Gaussian" systems have the highest spatial resolution, but lowest throughput of all probe forming systems due to the serial exposure of patterns one pixel at a time. They have, however, the advantage that corrections can be applied dynamically and pixel by pixel to compensate for aberrations of the electron lenses and deflection units in the system.
An increase in throughput is effected by producing a larger spot on the wafer, adjustable in size, so that it is equal to the linewidth of the circuit. Those more efficient, i.e. higher throughput systems use a shaped spot on the wafer by generating an image of an aperture or other object illuminated by the source, not of the source itself. The image is electronically variable in size, adjustable to compose a pattern feature with serial exposures projecting up to several hundred pixels in parallel. An example is disclosed in U.S. Pat. No. 4,243,866.
The highest throughput is obtained with a projection system that projects all pixels in parallel. The classic e-beam projection system is modelled on optical projection systems. In the foreseeable future, chips may have a size of approximately 17 mm.times.35 mm, so that at a typical 4:1 demagnification ratio, the reticle will have a size of 70 mm.times.140 mm. Current technology is unable to produce an electron lens that will cover that size reticle with an acceptable fidelity at a nominal device groundrule corresponding to 0.25 .mu.m critical dimension.
Throughput is essential if e-beam systems are to compete with light optical and X-ray systems. Therefore, mask projection would be the technique of choice for wafer exposure. A key requirement for high throughput is, of course, a highly intense beam. High power means great heat load on the reticle, which would lead to intolerable distortion of the reticle. An alternative approach to minimize thermal distortion of the reticle of a projection system is that of using a scattering reticle, as described in S. D. Berger & J. M. Gibson, APPL. PHYS. LETTERS 57 (2) (1990) 153), instead of an absorbing reticle. A scattering reticle requires an aperture above the wafer that preferentially absorbs scattered radiation having a greater scattering angle, thus translating scattering contrast into intensity contrast on the wafer.
A fundamental disadvantage of full field projection systems as compared with probe forming systems is the inability to dynamically correct for any aberrations (image blur, distortion) within a chip or exposure field, due to imperfections of the reticle, the e-beam system and/or the wafer.
Consequently, the art has long sought an e-beam system that would offer an acceptable tradeoff between accuracy and throughput.